Operating systems (OS) such as Microsoft Windows XP (a registered trademark) are provided with a password-protected screen saver. For a computer in which this kind of OS has been installed, the screen saver is activated to lock the computer, if no key-in or cursor action has been done for a certain period. After the computer is locked, before accepting a new key-in or cursor action attempted by its user, the computer prompts the user to enter a password. Only if the user has entered a valid password, the computer is unlocked (Windows XP Home Edition COMPLETE, pages 263-264, published by SYBEX Inc.). Thereby, the computer is protected from malicious access.